1. Field of the Invention
The present invention relates an optical wavelength converting device comprising a waveguide or core of a nonlinear optical material and a clad surrounding the core for converting a wavelength of a fundamental wave entering the waveguide to a 1/2 wavelength of a secondary harmonic, which utilizes the Second Harmonic Generation (SHG) in which the fundamental wave passing through the waveguide generates the secondary harmonic.
2. Description of the Related Art
There is an optical wavelength converter system of waveguide-type as shown in FIG. 1. This three-dimensional waveguide type wavelength converting converter 5 utilizes a semiconductor laser 1 as a light source for generating a fundamental wave as a light beam. The optical wavelength converter 5 comprises a core made of a nonlinear optical material and a clad substrate 3 surrounding the core. A converging lens 2 placed between the laser and the converter converges the fundamental wave beam into a rectangle end surface of the core which is an optical coupling portion. The converter 5 converts the wavelength of the fundamental wave to a 1/2 wavelength of secondary harmonic.
Conventional waveguide types of optical wavelength converting elements are generally classified into Cherenkov radiation type, Inverted domain type and Simple birefringence type. Almost all of the waveguide portions in these types are designed so as to satisfy single-mode guided conditions with respect to the fundamental wave as well as with respect to the secondary harmonic excepting the Cherenkov radiation type.
The single-mode waveguide has a small sectional area of the end surface and a small difference of refractive indexes between the waveguide and the substrate. An entering fundamental wave propagates with a single-mode in the waveguide. The electric susceptibility due to the secondary nonlinear optical effect of the nonlinear optical material generates a secondary harmonic, so that both the fundamental wave and secondary harmonic are mode-coupled and then the secondary harmonic is gradually converted into its single-mode.
In the conventional waveguide-type optical wavelength converters, a change of temperature and/or physical fluctuation in the optical coupling system propagating the fundamental wave changes a coupling rate of the fundamental wave, so that the output of the secondary harmonic is apt to be unstable. Since the sectional area of the waveguide is small, the power density of the injected fundamental wave becomes high, so that the waveguide may be broken due to the high power density. If the sectional area of the waveguide is expanded in order to obtain the single-mode condition, then an index difference between both the waveguide and substrate becomes very small, so that the difficulty of control and selection of materials of both the waveguide and substrate increases the complication in the production of the optical wavelength converter.
It is impossible to select the injected fundamental wave mode and only the single-mode of fundamental wave is used for the optical wavelength conversion. Therefore, the conventional optical wavelength converter outputs only a single transverse mode of secondary harmonic.